1. Field of the Invention
The present invention relates to a scanning optical system, an optical scanner, an image forming apparatus, and a color image forming apparatus.
2. Description of the Related Art
An optical scanner is widely used in a digital copying machine, a laser printer or the like. A scanning optical system is an optical system that converges a luminous flux deflected by an optical deflector on a scan target surface as a light spot, and is used in the optical scanner. The scanning optical system is required to have high optical performances including convergence of a luminous flux on an entire scan range of the scan target surface as light spots, highly accurate constant velocity function for scanning the entire scan target surface at constant velocity by the luminous flux deflected at constant angular velocity, and satisfactory corrections of various aberrations. Conventionally, the scanning optical system of the scanner is normally constituted by a lens system. The number of lenses varies from one to two or more. In terms of cost efficiency, the scanning optical system constituted by one lens is advantageous. However, the scanning optical system constituted by one lens has extremely small in the number of design parameters as compared with a scanning optical system constituted by more than one lens. In addition, if the scanning optical system is constituted by one lens of a simple lens surface shape, it is difficult to ensure satisfactory aberration correction.
There is proposed a technique for ensuring satisfactorily correcting aberrations and constant velocity characteristics by adopting a special lens surface shape even if the scanning optical system is constituted by a single lens (Japanese Patent Application Laid-open (JP-A) Nos. H10-253915 and 2002-90677). The technology disclosed in JP-A No. H10-253915 is characterized as follows. If a direction of a main beam of a deflected luminous flux, when the main beam is orthogonal to a scan target surface, is assumed as a reference direction X, an angle θ1 of the main beam of the deflected luminous flux directed toward one end A of an effective write area with respect to the reference direction X differs from an angle θ2 of the main beam of the deflected light beam directed toward the other end B of the effective write area with respect to the reference direction X. In addition, at least one scan imaging element is shifted or tilted from the reference direction X so as to lessen the influence of sag. The technology disclosed in JP-A No. 2002-90677 is characterized by setting a paraxial radius of curvature within a main scanning plane, an aspherical amount, a distance to a scan target surface, and the like of a single lens that constitutes a scan imaging element to satisfy predetermined conditions.
Recently, there is a strong demand for improving a scan density of the optical scanner. To meet this demand, reduction of a diameter of a light spot and stabilization of the light spot are required. However, to do so, it is insufficient to correct a geometric optical aberration but it is important and necessary to correct a wave optical aberration. The technologies disclosed in JP-A Nos. H10-253915 and 2002-90677 does not refer to correction of such a wave optical aberration.
It is also required to scale down the optical scanner. The scale down of the optical scanner includes a reduction in a size of the scanner and a reduction in a thickness of the lens. However, if the optical scanner is scaled down while maintaining its performances, a tolerance is narrowed so as to increase magnification is imposed on the optical scanner. Accordingly, it is desired to realize a scanning optical system capable of ensuring high performances with a shape as simple as possible.